Chapter 8
Phase3: Gaining Access Using
Network Attacks
Tools used in Network Attacks
 Sniffing
 Spoofing
 Session hijacking
 Netcat
Sniffer
 Allows attacker to see everything sent across the
network, including userIDs and passwords
 NIC placed in promiscuous mode
 Tcpdump http://www.tcpdump.org
 Windump http://netgroup-serv.polito.it/windump
 Snort http://www.snort.org
 Ethereal http://www.ethereal.com
 Sniffit
http://reptile.rug.ac.be/~coder/sniffit/sniffit.html
 Dsniff http://www.monkey.org/~dugsong/dsniff
Island Hopping Attack
 Attacker initially takes over a machine via
some exploit
 Attacker installs a sniffer to capture userIDs
and passwords to take over other machines
Figure 8.1 An island hopping attack
Passive Sniffers
 Sniffers that passively wait for traffic to be
sent to them
 Well suited for hub environment
 Snort
 Sniffit
Figure 8.2 A LAN implemented with a hub
Sniffit in Interactive Mode
 Useful for monitoring session-oriented
applications such as telnet, rlogin, and ftp
 Activated by starting sniffit with “-i” option
 Sorts packets into sessions based on IP addresses
and port numbers
 Identifies userIDs and passwords
 Allows attacker to watch keystrokes of victim in
real time.
 http://reptile.rug.ac.be/~coder/sniffit/sniffit.html
Figure 8.3 Using Sniffit in interactive mode to
sniff a userID and password
Switched Ethernet LANs
 Forwards network packets based on the
destination MAC address in the Ethernet
header
 Renders passive sniffers ineffective
Figure 8.4 A LAN implemented with a switch
Figure 8.5 A switched LAN prevents an
attacker from passively sniffing traffic
Active Sniffers
 Effective in sniffing switched LANs
 Injects traffic into the LAN to redirect
victim’s traffic to attacker
Dsniff
 Active sniffer
 http://www.monkey.org/~dugsong/dsniff
 Runs on Linux, Solaris, OpenBSD
 Excels at decoding a large number of
Application level protocols
– FTP, telnet, SMTP, HTTP, POP, NTTP, IMAP,
SNMP, LDAP, Rlogin, RIP, OSPF, NFS, NIS,
SOCKS, X11, IRC, ICQ, Napster, MS SMB,
and SQL
 Performs active sniffing using MAC
flooding or arpspoof
Dsniff’s MAC Flooding
 Initiated via Dsniff’s Macof program
 Foul up switches by sending out a flood of
packets with random MAC addresses
 When switch’s memory becomes full, the
switch will start forwarding data to all links
on the switch
 At this point, Dsniff or any passive sniffer
can capture desired packets
Dsniff’s Arpspoof
 Used in switched environment where MAC
flooding does not work
 defeats switches via spoofed ARP messages
 Attacker’s machine initially configured with
“IP forwarding” to forward incoming
network traffic to a default router
 Dsniff’s arpspoof program activated to send
fake ARP replies to the victim’s machine to
poison its ARP table
 Attacker can now sniff all traffic on the
LAN
Figure 8.6 Arpspoof redirects traffic, allowing
the attacker to sniff a switched LAN
Dsniff’s DNSspoof
 redirects traffic by sending false DNS
information to victim
 Attacker initially activates arpspoof and
dnsspoof
 When victim tries to browse a web site, a
DNS query is sent but the attacker sends a
poisoned DNS response
 Victim unknowingly communicates with
another web server
Figure 8.7 A DNS attack using Dsniff
Sniffing HTTPS and SSH
 Security is built on a trust model of underlying
public keys
– HTTPS server sends to browser a certificate containing
server’s public key signed by a Certificate Authority
– SSL connection uses a session key randomly generated by
server to encrypt data between server and client
– With SSH, a session key is transmitted in an encrypted
fashion using a private key stored on the server
 Dsniff takes advantage of poor trust decisions made
by a clueless user via man-in-the middle attack
– Web browser user may trust a certificate that is not signed
by a trusted party
– SSH user can still connect to a server whose public key
has changed
Attacking HTTPS and SSH
via Dsniff
 Webmitm
 Sshmitm
Figure 8.8 In a person-in-the-middle attack, the attacker
can grab or alter traffic between Alice and Bob
Dsniff’s Webmitm
 Program used to proxy all HTTP and
HTTPS traffic
 acting as an SSL proxy, webmitm can
establish two separate SSL connections
– One connection between victim and attacker
– One connection between attacker and web
server
 Webmitm sends attacker’s certificate to
victim
Figure 8.9 Sniffing an HTTPS connection using dsniff’s
person-in-the-middle attack
Figure 8.10 Netscape’s warning messages for SSL
connections using certificates that aren’t trusted
Figure 8.11 Internet Explorer’s warning messages are
better, but not by much
Figure 8.12 Webmitm’s output shows entire content of SSLencrypted session, including the userID and password
Dsniff’s sshmitm
 Allows attacker to view data sent across an
SSH session
 Supports sniffing of SSH protocol version 1
Dsniff’s other Tools
 Tcpkill
– Kills an active TCP connection.
– Allows attacker to sniff the UserID and
password on subsequent session
 Tcpnice
– Slows down traffic by injecting tiny TCP
window advertisements and ICMP source
quench packets so that sniffer can keep up with
the data
 Filesnarf
– Grabs files transmitted using NFS
Dsniff’s other Tools (cont.)
 Mailsnarf
– Grabs email sent using SMTP and POP
 Msgsnarf
– Grabs messages sent using AOL Instant
Messenger, ICQ, IRC, and Yahoo Messenger
 URLsnarf
– Grabs a list of all URLs from HTTP traffic
 Webspy
– Allows attacker to view all web pages viewed
by victim
Sniffing Defenses
 Use HTTPS for encrypted web traffic
 Use SSH for encrypted login sessions
– Avoid using Telnet
 Use S/MIME or PGP for encrypted email
 Pay attention to warning messages on your
browser and SSH client
 Configuring Ethernet switch with MAC
address of machine using that port to
prevent MAC flooding and arpspoofing
 Use static ARP tables on the end systems
IP Address Spoofing
 Changing or disguising the source IP
address
 used by Nmap in decoy mode
 Used by Dsniff in dnsspoof attack
– DNS response sent by Dsniff contains source
address of the DNS server
 Used in denial-of-service attacks
 Used in undermining Unix r-commands
 Used with source routing attacks
Simple IP Address Spoofing
 Pros
– Works well in hiding source of a packet flood
or other denial-of-service attack
 Cons
– Difficult for attacker to monitor response
packets
– Any response packet will be sent to spoofed IP
address
– Difficult to IP address spoof against any TCPbased service unless machines are on same
LAN and ARP spoof is used
Figure 8.13 The TCP three-way handshake inhibits
simple spoofing
Undermining Unix r-commands
via IP Address Spoofing
 When one Unix system trusts another, a user can
log into the trusted machine and then access the
trusting machine without supplying a password by
using rlogin, rsh, and rcp
 hosts.equiv or .rhosts files used to implement
trusts
 IP address of trusted system used as weak form of
authentication
 Attacker spoofing IP address of trusted system can
connect to trusting system without providing
password
 “Rbone” tool http://packetstorm.security.com
Figure 8.14 Bob trusts Alice
Figure 8.15 Everyone trusts Alice, the administrator’s
main management system
Spoofing Attack against Unix Trust
Relationships
1.
2.
3.
4.
5.
6.
Attacker interacts with targeted trusting server to
determine predictability of initial sequence number
Attacker launches a denial-of-service attack (eg. SYN
flood or smurf attack) against trusted system to force it
not to respond to a spoofed TCP connection
Attacker rsh to targeted trusting server using spoofed IP
address of trusted server
Trusting server sends an SYN-ACK packet to the
unresponsive trusted server
Attacker sends an ACK packet to trusting server with a
guess at the sequence number. If ISN is correct, a
connection is made.
Although attacker cannot initially see reply packets from
trusting server, attacker can issue command to append
“++” to hosts.equiv or .rhosts file. Trusting server will
Figure 8.16 Spoofing attack against Unix trust
relationships
Spoofing with Source Routing
 Works if routers support source routing
 Attacker generates TCP SYN packet destined for
trusting server containing spoofed IP address of
trusted machine and fake source route in IP header
 Trusting server will reply with a SYN-ACK
packet containing a source route from trusting
server to attacker to trusted machine.
 Attacker receives the reply but does not forward it
to the trusted machine.
 Attacker can pose as trusted machine and have
interactive sessions with trusting machine
Figure 8.17 Spoofing attack using source routing
IP Spoofing Defenses
 Make sure that initial sequence numbers generated
by TCP stacks are difficult to predict
– Apply latest set of security patches from OS vendor
– Used Nmap to verify predictability of ISN
 Use ssh instead of r-commands
 Avoid applications that use IP addresses for
authentication
– Authentication should use passwords, PKI, or Kerberos
or other methods that tie a session back to a user.
 Use “anti-spoof” packet filters at border routers
and firewalls
– ingress (incoming) and egress (outgoing) filters
 Block source-routed packets on routers
– “no ip sourceroute”
Figure 8.18 Anti-spoof filters
Network-based Session Hijacking
 Attack based on sniffing and spoofing
 Occurs when attacker steals user session such as
telent, rlogin, or FTP.
– Innocent user thinks that his session was lost, not stolen
 Attacker sits on a network segment where traffic
between victim and server can be seen
 Attacker injects spoofed packets contain source IP
address of victim with proper TCP sequence
numbers
 If hijack is successful, server will obey all
commands sent by attacker.
 May cause ACK storm between victim and server
when victim tries to resynchronize its sequence
number
Figure 8.19 A network-based session hijacking
scenario
Figure 8.20 An ACK storm triggered by session
hijacking
Host-based Session Hijacking
 Attacker can hijack a session on source or
destination machine if he has super user privileges
on that machine
 Highjacking tool allows attacker to interact with
the local terminal devices (tty)
 Attacker can read all session information from
victim’s tty
 Attacker can control victim’s tty by injecting
keystrokes into the tty
 Host-based session hijacking preferable to
network-based session hijacking if target machine
is already compromised
Session Hijacking Tools
 Network-based
– Hunt http://www.cri.cz/kra/index.html
– Dsniff’s sshmitm tool in –I mode
– Juggernaut http://packetstorm.securify.com
 Host-based
– TTYWatcher
http://ftp.cerias.purdue.edu/pub/tools/unix/sysut
ils
– TTYSnoop http://packetstorm.securify.com
Session Hijacking with Hunt
 Runs on Linux platform
 Allows attacker to see many sessions going across the
network and to hijack a particular session
 ACK storm may occurs after attacker injects one or two
commands
 ACK storm can be prevented running Hunt in a mode
supporting ARP spoofing
– Don’t want victim’s packets to be seen by server and visa versa
– Attacker sends bogus ARP replies to both victim and server to
poison their ARP tables
 Attacker sees traffic sent by victim and server
 Hunt can resynchronize the connection so session can be
returned to victim
– Message is sent to victim to type certain number of keys to
increment victim’s sequence number
Figure 8.21 Avoiding the ACK storm by ARP spoofing
Figure 8.22 The attacker’s view of a session hijacking
attack using Hunt
Figure 8.23 By ARP spoofing two routers between Alice and
Bob, all traffic between the routers (including the traffic
between Alice and Bob) will be directed through Eve
Session Hijacking Defenses
 Use SSH or VPN for securing sessions
– Attackers will not have the keys to encrypt or
decrypt traffic
– Pay attention to warning messages about any
change of public key on server since this may
be a person-in-the-middle attack
Netcat
 Network version of “cat” utility
 Allows user to move data across a network using
any TCP or UDP port
 Runs on both Unix and Windows NT
 http://www.l0pht.com/~weld/netcat/
 Netcat executable “nc” operates in two modes
– Client mode allows user to initiate connection to any
TCP or UDP on a remote machine and to take input
data from standard input (eg keyboard or output of
pipe)
– Listen mode (-l option) opens any specified TCP or
UDP port on local system and waits for incoming
connection and data through port. Data collected is sent
to standard output (eg. Screen or input of pipe)
Figure 8.24 Netcat in client mode and listen mode
Netcat for File Transfer
 Useful for transfering files in/out networks
which block FTP sessions
 File can be transferred by having netcat
client either “push” it or “pull” it
Figure 8.25 Pushing a file across the network using Netcat
Figure 8.26 Pulling a file across the network using Netcat
Netcat for Port Scanning
Note: verbose option will cause Netcat to display a
list of open ports on target machine
Connecting to Open UDP and TCP Ports
via Netcat
Vulnerability Scanning
using Netcat
 Finds RPC vulnerabilities
 Finds NFS exports whose file systems can
be viewed by everyone
 finds machines with weak trust relationship
 Finds machines with very weak passwords
 Finds buggy FTP servers
 Vulnerability scanning is limited compared
to Nessus
Using Netcat to Create a Passive Backdoor
Command Shell on any UDP/TCP port
Using Netcat to Actively Push a Backdoor
Command Shell to Attacker
Note: useful if firewall blocks inbound connections from
Internet
Relaying Traffic with Netcat
 Netcat can be used to bounce an attack
across may machines controlled by an
attacker
 One each relay machine, a Netcat listener is
configured to forward network traffic to a
Netcat client on same machine
 Netcat client is configured to forward data
to another machine in the relay
 Difficult to trace attacker especially if relays
cross language and political borders
Figure 8.27 Setting up relays using Netcat
Figure 8.28 Directing traffic around a packet filter,
using a Netcat relay
Creating a Netcat Relay
 Modifying “inetd.conf” or
 Setting up a backpipe
Using Inetd to create a Netcat Relay
Note: /etc/inetd.conf configured to have Netcat listen on port
11111 and forward traffic to port 54321 on host next_hop
Using a Backpipe to create a Netcat Relay
Note: Netcat setup to listen on port 1111, forwarding data to
next_hop on port 54321. The backpipe file is used to direct
response traffic back from destination to the source
Netcat Defenses
 Configure firewall to limit
incoming/outgoing traffic to applications
(eg. DNS, email, WWW, FTP) that have a
business need
 Systems should be listening only on ports
that have a business need
 Systems should have the latest security
patches
 Know what process are commonly running
on your systems so that you can rogue
server process